Method of making a surface acoustic wave device

ABSTRACT

A method of producing a SAW (Surface Acoustic Wave) device, including the steps of implanting ions in an entire surface of a piezoelectric member of the SAW device, so that an ion implantation layer is formed therein; performing a heat treatment of the piezoelectric member, so that a heat-treated ion implantation layer is formed in the entire surface of the piezoelectric member; and providing an electrode of a comb type resonator on the heat-treated ion implantation layer.

Division of prior application No. 08/615,798, filed Mar. 14, 1996, nowU.S. Pat. No. 5,796,205, which is a continuing application ofapplication No. 08/290,639 filed Aug. 16, 1994, abandoned

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surface acoustic wave devices whichutilizes a surface acoustic wave propagating through a surface of apiezoelectric substrate or a surface of a piezoelectric thin filmprovided on a predetermined base.

Recently, down-sizing of mobile communications devices such as portabletelephone sets has been facilitated progressively, and there has been ademand for down-sizing of parts and improvements in the performancethereof. Normally, these communications devices are equipped withelements such as an oscillator, filter and a wave distributing elementused to generate and branch a signal. Recently, there has beenconsiderable activity in the research and development of devices using asurface acoustic wave in order to facilitate down-sizing of the aboveelements. Particularly, a surface acoustic wave filter in which aninductance is added to a surface acoustic wave resonator equipped with areflector has a low insertion loss, and enables broadening of the bandand high suppression performance. Hence, applications of such a surfaceacoustic wave filter to the next-generation automobile telephone setsand portable telephone sets have been considered.

2. Description of the Related Art

Surface acoustic wave (hereinafter simply referred to as SAW) devicesuse a surface acoustic wave that propagates through a surface of adielectric body cut out of a piezoelectric crystal or a surface of apiezoelectric thin film. It is possible to control (adjust)characteristics of such SAW devices such as the central frequency andthe pass-band range by changing the propagation speed of the SAW and/orthe electromechanical coupling factor. The following first through thirdmethods are known as means for controlling the characteristics of theSAW devices.

The first method is to select an appropriate piezoelectric crystalmaterial, an appropriate cut surface of the crystal and/or the SAWpropagating direction. For example, a crystal material such as LiNbO₃ orLiTaO₃ is used, and the X-112° cut or 36° Y-X cut surface of the LiTaO₃crystal is used.

The second method is shown in FIG. 1A. As shown in FIG. 1A, the secondmethod is to provide an insulating film 3 between the surface of apiezoelectric substrate 1 (or the surface of a piezoelectric thin film)and a comb-type electrode 2 (see Japanese Laid-Open Patent ApplicationNo. 48-26452 or No. 52-16146). FIG. 1B shows a variation of the secondmethod. In FIG. 1B, an insulating film 4 is formed on the surface of thepiezoelectric substrate 1 (or the surface of the piezoelectric thinfilm) on which the comb-type electrode 2 is formed.

The third method is shown in FIG. 1C, in which an ion implantation layer5 containing Ar, O₂ or Si ions is formed in the surface of thepiezoelectric substrate (or the surface of the piezoelectric thin film).The third method is disclosed in, for example, Japanese Laid-Open PatentApplication No. 63-169806.

The first method changes not only the SAW propagation speed and theelectromechanical coupling factor but also the SAW mode and thetemperature coefficient of the SAW device. Hence, it is very difficultto optimize these parameters and thus produce devices having the desiredcharacteristics.

The second method shown in FIG. 1A has a disadvantage found by anexperiment conducted by the inventors. In the experiment, the insulatingfilm 3 made of SiO₂ (which is the most general insulating material) wasformed between the surface of the piezoelectric substrate 1 and thecomb-type electrode 2. FIG. 2 shows the result of the experiment, inwhich the horizontal axis denotes the thickness of the SiO₂ insulatingfilm 3 and the vertical axis denotes the electromechanical couplingfactor k² (%). The electromechanical coupling factor greatly depends onthe thickness of the SiO₂ film 3, particularly when the thickness of theSiO₂ film 3 is equal or less than 1000 Å. In other words, the value ofthe electromechanical coupling factor changes greatly due to even asmall variation in the thickness of the SiO₂ film 3, and barely changeswhen the thickness of the SiO₂ film is greater than 1000 Å. Hence, thesetting of the thickness of the SiO₂ film is very difficult. Further,absorption and damping of the SAW are caused by the SiO₂ film, and hencethere is a large insertion loss of a SAW device to which the secondmethod is applied.

The variation of the second method shown in FIG. 1B has disadvantagessimilar to those of the second method.

The third method has a disadvantage in that the SAW propagation speed ischanged by the thermal history resulting from a process carried outafter the third method. Hence, the third method does not have goodreliability, and the insertion loss of a SAW device to which the thirdmethod is applied is not negligible.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a SAW deviceand method of producing the same in which the above disadvantages areeliminated.

A more specific object of the present invention is to provide a SAWdevice and method in which various parameters of the SAW device such asthe SAW propagation speed and the electromechanical coupling factor canbe easily controlled and there is a reduced insertion loss.

The above objects of the present invention are achieved by a SAW(Surface Acoustic Wave) device comprising: a piezoelectric member; aheat-treated ion implantation layer formed in a surface portion of thepiezoelectric member; and an electrode of a comb type resonator withreflectors provided on the heat-treated ion implantation layer.

The above objects of the present invention are also achieved by a methodof producing a SAW (Surface Acoustic Wave) device, the step comprisingthe steps of: a) implanting ions in a surface portion of a piezoelectricmember of the SAW device, so that an ion implantation layer is formedtherein; b) performing a heat treatment of the piezoelectric member, sothat a heat-treated ion implantation layer is formed in the surfaceportion of the piezoelectric member; and c) providing an electrode of acomb type resonator on the heat-treated ion implantation layer.

The above objects of the present invention are also achieved by a SAW(Surface Acoustic Wave) device comprising: a first resonator formed on apiezoelectric member and provided in a parallel arm;

and a second resonator formed on the piezoelectric member and providedin a series arm, the first resonator having a resonance frequencyapproximately equal to an anti-resonance frequency of the secondresonator, the piezoelectric member including an ion implantation layerformed in a surface portion of the piezoelectric member on which atleast one of the first resonator and the second resonator is provided.

The above objects of the present invention are also achieved by a methodof producing a SAW (Surface Acoustic Wave) device including a firstresonator formed on a piezoelectric member and provided in a parallelarm, and a second resonator formed on the piezoelectric member andprovided in a series arm, the first resonator having a resonancefrequency approximately equal to an anti-resonance frequency of thesecond resonator, the method comprising the steps of: a) implanting ionsin a surface portion of the piezoelectric member so that an ionimplantation layer is formed therein; and b) forming electrodes of acomb type of the first and second resonators on the piezoelectric memberso that at least one of the first and second resonators is provided onthe ion implantation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIGS. 1A, 1B and 1C are cross-sectional views of conventional methodsfor controlling the SAW propagation speed and the electromechanicalcoupling factor;

FIG. 2 is a graph of a relationship between the electromechanicalcoupling factor and the thickness of an SiO₂ film shown in FIG. 1A;

FIGS. 3A, 3B, 3C and 3D are cross-sectional views of a first embodimentof the present invention;

FIG. 4 is a plan view of a SAW device sample used to measure the surfaceacoustic velocity of a surface acoustic wave;

FIG. 5 is a graph of SAW propagation speeds of four samples each havingthe structure shown in FIG. 4;

FIG. 6 is a graph of the central frequency characteristic, insertionloss characteristic and pass-band range characteristic of a SAW filteras a function of a dose of Ar ions;

FIGS. 7A and 7B are diagrams for explaining a pitch of a comb-typeelectrode having a resonator structure;

FIG. 8A is an equivalent circuit diagram of a structure shown in FIG.17;

FIG. 8B is a diagram of an electrode used in each resonator shown inFIG. 17;

FIG. 8C is a graph of a resonator characteristic of the resonator shownin FIG. 8B;

FIG. 9 is a graph of the pass-band characteristic of a ladder-type SAWfilter;

FIG. 10 is a graph of the central frequency and shape factor of aladder-type SAW filter as a function of the heat treatment temperature;

FIG. 11 is a graph for explaining the shape factor of the SAW filter;

FIG. 12 is a graph of a locking curve used to measure the depth of ionimplantation;

FIG. 13 is a graph of the insertion loss characteristic and filter shapefactor characteristic as a function of the depth of ion implantation;

FIGS. 14A, 14B, 14C, 14D, 14E and 14F are graphs the pass-bandcharacteristics of samples in which different doses of ions areimplanted;

FIGS. 15A and 15B are cross-sectional views of means for preventingpyroelectricity caused when implanting ions;

FIGS. 16A and 16B are cross-sectional views of another means forpreventing pyroelectricity caused when implanting ions;

FIG. 17 is a plan view of a SAW filter according to a second embodimentof the present invention;

FIGS. 18A, 18B, 18C and 18D are diagrams of a method of producing theSAW filter shown in FIG. 17;

FIG. 19 is a plan view of a practical example of the SAW filter shown inFIG. 17;

FIG. 20 is an equivalent circuit diagram of the SAW filter shown in FIG.19;

FIG. 21 is a plan view of a SAW filter according to a third embodimentof the present invention;

FIGS. 22A and 22B are plan views of comb-type electrodes of a resonatorstructure used in the SAW filter shown in FIG. 21;

FIG. 23 is a graph of the pass-band characteristic of the SAW filtershown in FIG. 21;

FIG. 24 is a plan view of a practical example of the SAW filteraccording to the present invention;

FIG. 25 is a cross-sectional view taken along line XXV--XXV shown inFIG. 24;

FIG. 26 is an equivalent circuit diagram of the SAW filter shown in FIG.24; and

FIG. 27 is a perspective view of another practical example of the SAWfilter according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to FIGS. 3A through 3D,of a first embodiment of the present invention.

Referring to FIG. 3A, a substance for controlling the SAW propagationspeed and the electromechanical coupling factor is ion-implanted in asurface portion of a piezoelectric substrate (or a piezoelectric thinfilm formed on a predetermined base) 1, so that an ion implantationlayer 11 is formed in the surface portion of the piezoelectricsubstrate 1. Examples of such a substance is boron (B), argon (Ar),fluorine (F), neon (Ne), hydrogen (H) or phosphorus (P).

Next, a heat treatment (annealing) for controlling the SAW propagationspeed and the electromechanical coupling factor, so that a heattreatment layer 12 for the ion implantation layer 11 is formed, as shownin FIG. 3B. For example, the heat treatment heats the piezoelectricsubstrate 1 at 350° C. for 30 minutes in vacuum.

Then, as shown in FIG. 3C, a metallic layer 13 is coated on the heattreatment layer 12. Thereafter, as shown in FIG. 3D, unnecessaryportions of the metallic layer 13 are removed, so that comb-typeelectrodes (interdigital electrodes) 14 having the function ofgenerating a surface acoustic wave and outputting an electric signalcorresponding to a surface acoustic wave are defined on the surface ofthe heat treatment layer 12.

The first embodiment of the present invention will be further describedwith reference to FIGS. 4, 5, 6A and 6B.

FIG. 4 shows a sample 20 used to measure the SAW propagation speed(surface acoustic velocity). The sample 20 has the substrate 1 cut outof a LiTaO₃ crystal. Interdigital electrodes 14₁, 14₂, 14₃ and 14₄ and ametallic pattern 15 are formed on the surface of the substrate 1. Themetallic pattern 15 is interposed between the opposite electrodes 14₁and 14₂.

The interdigital electrodes 14₁, 14₂, 14₃ and 14₄ and the metallicpattern 15 are approximately 3000 Å thick, and are defined by patterningan Al-Cu film by the photolithograph technique. Each of the interdigitalelectrodes 14₁, 14₂, 14₃ and 14₄ includes a pair of electrodes of acomb-type resonator as shown in FIG. 7A, and the pitch of the adjacentfinger portions is a predetermined value as shown in FIG. 7B. Inputterminals 16 are respectively connected to the interdigital electrodes14₁ and 14₃, which function as input electrodes. A ground terminal 17 isconnected in common to the interdigital electrodes 14₂ and 14₄. Outputterminals 18 are respectively connected to the electrodes 14₂ and 14₄,which function as output lectrodes. A ground terminal 19 is connected incommon to the interdigital electrodes 14₂ and 14₄.

In the measurement of the SAW propagation speed, the following fourdifferent samples were prepared. The first sample had the piezoelectricsubstrate 1 in which no ions were implanted. The second sample had thepiezoelectric substrate 1 in which boron ions were implanted at 180 keVand a dose of 1×10¹⁴ ions/cm². The third sample was formed by applying aheat treatment to the second sample in which the second sample washeated at 200° C. for 10 minutes in a vacuum of approximately 2×10⁻⁸Torr. The fourth sample was formed by applying a heat treatment to thesecond sample in which the second sample was heated at 350° C. for 30minutes in a vacuum of 2×10⁻⁸ Torr. Then, the interdigital electrodes14₁, 14₂, 13₃ and 14₄ and the metallic pattern 15 were formed on thefirst through fourth samples under the same condition.

FIG. 5 is a graph of the SAW propagation speeds of the four samples, inwhich the horizontal axis denotes h/λ where h is the thickness of themetallic layer 15 and λ is the electrode pitch of the interdigitalelectrodes 14₁, 14₂, 13₃ and 14₄, and the vertical axis denotes Vo/Vmwhere Vm is the surface acoustic velocity between the interdigitalelectrodes 14₁ and 14₂ under metal and Vo is the surface acousticvelocity between the interdigital electrodes 14₃ and 14₄ on freesurface.

In FIG. 5, symbol "·" denotes plotted measured values of the SAWpropagation speed of the first sample, and symbol "×" denotes plottedmeasured values of the SAW propagation speed of the second sample.Symbol "Δ" denotes plotted measured values of the SAW propagation speedof the third sample, and symbol "∘" denotes plotted measured values ofthe SAW propagation speed of the fourth sample.

It can be seen from FIG. 5 that the ratio Vo/Vm decreases by ionimplantation and accordingly the electromechanical coupling factordecreases.

The ratio Vo/Vm of the third sample obtained by heating the secondsample at 200° C. for 10 minutes in a vacuum of approximately 2×10⁻⁸Torr is increased so that it is located between the ratio Vo/Vm of thefirst sample and the ratio Vo/Vm of the second sample.

It can be seen from FIG. 5 that the ratio Vo/Vm of the fourth sampleobtained by heating the second sample at 350° C. for 30 minutes in avacuum of approximately 2×10⁻⁸ Torr is increased so that it isapproximately equal to the ratio Vo/Vm of the first sample.

It is to be noted that the inventors found through the experiments asshown in FIG. 5 that almost the same relationship between Vo/Vm and h/λas that of the sample in which no ions are implanted can be obtained byperforming the heat treatment (equal to or higher than 350° C.) on thesamples in which ions are implanted and that the present invention wasmade based on the new facts found by the inventors.

FIG. 6 is a graph of filter characteristics of a SAW filter. In each ofthe graphs of FIG. 6, the vertical axes denote the central frequency(MHz), insertion loss (dB) and the band range (MHz) of the SAW filter,and the horizontal axis denotes the dose of Ar ions (ions/cm²) implantedin the substrate of the SAW device.

In the measurement, five different types of samples of the SAW filterwere prepared in which the substrate thereof was a single crystal ofLiTaO₃. The first sample has the LiTaO₃ substrate in which no ions wereimplanted (a dose is zero: measured values are plotted by symbol "·".The second sample has the LiTaO₃ substrate in which Ar ions wereimplanted at 180 keV and a dose of 1×10¹⁴ ions/cm². The third sample hasthe LiTaO₃ substrate in which Ar ions were implanted at 180 keV and adose of 1×10¹⁶ ions/cm². The measured values of the second and thirdsamples are lotted by symbol "∘" in FIG. 6. The fourth sample was formedby heating the second sample at 300° C. for 30 minutes. The fifth samplewas formed by heating the third sample at 300° C. for 30 minutes. Themeasured values of the fourth and fifth samples are plotted by symbol"×" in FIG. 6.

The central frequency of the second sample (Ar ions are implanted at adose of 1×10¹⁴ ions/cm²) becomes 55 MHz (5.9%) lower than the centralfrequency of the first sample (no Ar ions are implanted) by the ionimplantation. The central frequency of the fourth sample is slightlyrestored by the heat treatment and is 42 MHz (4.5%) lower than thecentral frequency of the first sample.

The band width of the second sample becomes 26 MHz (41%) narrower thanthe band width of the first sample by the ion implantation. The bandwidth of the fourth sample is slightly restored by the heat treatmentand is 20 MHz (32%) narrower than the band width of the first sample.

The insertion loss of the second sample is degraded by 2.3 dB (2.3%) bythe ion implantation. The insertion loss of the fourth sample isslightly restored by the terminal treatment and is 0.6 dB (0.6%) greaterthan that of the fourth sample.

It can be seen from FIG. 6 that by the heat treatment, thecharacteristics of the SAW filter in which ions are implanted becomesclose to those thereof in which no ions are implanted. It can also beseen from FIG. 6 that as the dose of implanted ions increases, thecentral frequency of the SAW filter more greatly deviates from that ofthe SAW filter in which no ions are implanted.

With the above in mind, a SAW filter 21 of a ladder type shown in FIGS.8A and 8B will be considered. The SAW filter 21 includes fourone-terminal-pair (one port) SAW resonators R1, R2, R3 and 4 andinductance elements L1 and L2. The resonators R1 and R2 are connected inseries between a pair of terminals 22 and 23 by means of conductors 26,27 and 28 serving as series arms. A pair of terminals 24 and 25 isconnected together by a conductor 29. The resonator R3 and theinductance element L1 connected in series are connected to theconductors 26 and 29, respectively. The resonator R4 and the inductanceelement L2 connected in series are connected to the conductors 27 and29, respectively. In other words, the resonator R3 and the inductanceelement L1 are provided in a parallel arm, and the resonator R4 and theinductance element L2 are provided in another parallel arm.

As shown in FIG. 8B, each of the resonators R1, R2, R3 and R4 is made upof an excitation interdigital electrode 30 having two comb-typeelectrodes, and a pair of reflectors 31. The resonators R1, R2, R3 andR4 are formed on the ion-implantation layer obtained by implanting ionsin the surface portion of the piezoelectric substrate and carrying outthe heat treatment for the piezoelectric substrate according to theaforementioned method.

FIG. 8C is a graph of the resonance characteristic of the resonators inthe SAW filter shown in FIG. 8B. In the graph of FIG. 8C, the verticalaxis denotes attenuation (dB) and the horizontal axis denotes thefrequency (GHz). The solid line indicates the characteristic of the SAWresonators in which the heat treatment of the ion implantation layer hasbeen performed. The broken line indicates the characteristic of the SAWresonators in which no ions are implanted. The Q-value is defined asfollows:

    Q≡fo/Δf.sub.3dB

where fo is the frequency of the lowermost attenuation point, andf_(3dB) is the half-value width at the level 3 dB higher than the levelof the lowermost point.

Table shown below show the Q-values of the two resonatorcharacteristics.

                  TABLE                                                           ______________________________________                                                       Q value                                                        ______________________________________                                        No ion implantation                                                                            440                                                          Ion implantation &                                                                             550                                                          heat treatment                                                                ______________________________________                                    

It can be seen from the above Table that the ion implantation and thesubsequent heat treatment make the drop of the resonance characteristicshaper than that of the resonator characteristic of the SAW resonatorsin which no ions are implanted. By combining the resonators havingdifferent Q-values, it becomes possible to obtain the filtercharacteristics having a good shape factor (which will be described indetail later).

FIG. 9 is a graph of the band-pass characteristics of two samples. Oneof the samples (first sample) is a SAW resonator-type filter having ionimplantation layers. The other sample (second sample) is a SAWresonator-type which does not have any ion implantation layer. Thesolid-line curve shown in FIG. 9 indicates the band-pass characteristicof the first sample, and the broken-line curve shown therein indicatesthe band-pass characteristic of the second sample. The ion implantationof the first samples was carried out in such a way that double-charge Arions (Ar⁺⁺) were implanted in a LiTaO₃ (36° Y cut) substrate at 180 keVat a dose of 5×10¹³ ions/cm² and the thickness of the ion implantationlayer of the first sample was set to 2200 Å (h_(ion) /λ_(w)=0.22/2.7=0.081). The heat treatment subsequent to the above ionimplantation process was carried out according to the following steps(temperature profile) for the reason which will be described later:

1) Temperature rising speed from the room temperature to a predeterminedheat treatment temperature: 2° C. per minute;

2) Time period when the predetermined heat treatment temperature iskept: 1 hour; and

3) Temperature falling speed from the predetermined heat treatmenttemperature to the room temperature: 1° C. per minute.

It can be seen from FIG. 9 that the filter has a high Q-value and an20%-improved shape factor without degradation of the insertion loss andthe suppression performance.

FIG. 10 is a graph of the conditions of the heat treatment in which Arions are implanted in the piezoelectric substrate. The vertical axis ofthe graph of FIG. 10 denotes the central frequency (GHz) and the shapefactor of a SAW filter, and the horizontal axis denotes the heattreatment temperature (° C.).

The shape factor indicates the sharpness of the rise and fall of thewaveform of a signal in the pass-band. More particularly, referring toFIG. 11, the shape factor is defined by the ratio .sub.Δ f₂ /.sub.Δ f₁where .sub.Δ f₁ is the band width (f₂ -f₃) at an insertion loss of -3 dBand .sub.Δ f₂ is the band width (f₄ -f₅) at an insertion loss of -20 dB.The ideal value of the shape factor is 1. The value of the shape factorincreases, the sharpness of the rise and fall of the waveformdeteriorates. The shape factor of the SAW filter in which no ions areimplanted is equal to 2.3, while the shape factor is improved to 1.7 byimplanting ions in the above SAW filter as shown in FIG. 9.

The sample of the SAW filter used in the measurement related to thegraph of FIG. 10 had the piezoelectric substrate 1 cut out of a36°-Y-cut single crystal of LiTaO₃. Ar ions (Ar⁺⁺) in the double-chargestate were implanted in the piezoelectric substrate 1 at an accelerationenergy of 180 keV and a dose of 6×10¹³ ions/cm². The heat treatment wascarried out within the range of 120° C. to 600° C. It can be seen fromthe graph of FIG. 11 that the shape factor is stable between the heattreatment temperature between 120° C. and 600° C., and the centralfrequency is stable at heat treatment temperatures equal to or higherthan 350° C., preferably 400° C. It is also necessary for the heattreatment temperature to be less than the Curie temperature of thepiezoelectric substrate material. The Curie temperature of LiTaO₃ isapproximately 610° C.

In the heat treatment of the ion implantation layer 11, that is, theheat treatment at temperatures equal to or higher than 350° C., it ispreferable to set the heat treatment temperature as low as possible interms of productivity and operationability. In the embodiment beingconsidered, it is recommendable that the practical heat treatmenttemperature falls within the range of approximately 400° C. to 600° C.For example, when the piezoelectric substrate 1 is cut out of a36°-Y-cut single crystal of LiTaO₃, it is possible to set the heattreatment temperature within the range between 400° C. and 600° C.

There is a possibility that the piezoelectric substrate 1 may be damageddue to thermal shock in the heat treatment (heating and cooling) of theion implantation layer 11 formed in the piezoelectric substrate 1, whenrapidly heating or cooling the piezoelectric substrate 1. The inventorsrepeatedly conducted experiments in order to prevent the thermal shock,and found the following heat treatment conditions. The heat treatment inthe measurement shown in FIG. 10 was carried out under the followingconditions (temperature profile).

1) Temperature rising speed from the room temperature to a predeterminedheat treatment temperature: 3° C. per minute or lower;

2) Time period when the predetermined heat treatment temperature iskept: 30 minutes -1 hour; and

3) Temperature falling speed from the predetermined heat treatmenttemperature to the room temperature: 2° C. per minute or lower.

It is also possible to control the SAW propagation speed and theelectromechanical coupling factor by controlling the depth of ionimplantation (the thickness of the ion implantation layer) in order tocontrol the state of the crystal lattice in the surface portion of thepiezoelectric substrate. In order to inspect the relationship betweenthe depth of ion implantation and the SAW performance, a descriptionwill now be given of the method of measuring the depth of ionimplantation.

The depth of ion implantation can be measured by using a locking curveof double crystal X-ray diffraction for the ion implantation layer. FIG.12 shows a locking curve of a sample in which double-charge Ar ions(Ar⁺⁺) are implanted in an LiTaO₃ substrate at 180 keV and a dose of5×10¹³ ions/cm², the locking curve being obtained by using CuKα1X rays.In FIG. 12, a diffraction spectrum A located at the largest diffractionangle (left end in the drawing) results from the LiTaO₃ substrate. Otherdiffraction spectra result from the ion implantation layer. Thediffraction spectrum located at the lowest diffraction angle is theprimary maximum (B0), and the first (B1), second (B2), . . . andsecondary maximum diffraction spectra are observed in this order. Fromthe characteristic of the Laue function indicating the scatteringstrength of X rays, the thickness of the ion implantation layer iscalculated using the half-value width of the primary maximum (B0) shownin FIG. 12. The calculated thickness of the ion implantation layer isapproximately 2200 Å. In the above-mentioned manner, the depth of ionimplantation can be confirmed.

FIG. 13 is a graph of increase in the insertion loss (dB) and the filtershape factor of samples as a function of the thickness of the ionimplantation layer. The samples was formed by implanting double-chargedAr ions (Ar⁺⁺) into a LiTaO₃ (36° Y cut) at 180 keV and a dose of 5×10¹³ions/cm², and had different thicknesses of the ion implantation layers(which were confirmed by the aforementioned measurement method). In FIG.13, the vertical axes denote an increase in the insertion loss (dB) andthe filter shape factor, and the horizontal axis denotes the normalizeddepth (h_(ion) /λ_(w)) of the ion implantation obtained by normalizingthe depth hion by one wavelength λ_(w) of the surface acoustic wave.Normally, the surface acoustic wave propagates through a surface portionof the piezoelectric substrate within a depth equal to one wavelength ofthe surface acoustic wave. If ions are implanted in a depth greater thanone wavelength, the surface crystal portion is completely destroyed andthe propagation of the surface acoustic wave is affected. It can be seenfrom the graph of FIG. 13 that a good filter characteristic can beobtained when the filter shape factor (the ratio the band width at -1.5dB lower than the level at the least insertion loss to the band width at-21.5 dB lower than the level at the least insertion loss) is equal toor greater than 45% under the condition the increase in the insertionloss is equal to or less than 2 dB.

In order to obtain a filter shape factor of 45% or greater, the graph ofFIG. 13 shows that the thickness (h_(ion)) of the ion implantation layeris needed to satisfy the condition 0.07<h_(ion) /λ_(w) <0.33. Further,in order to obtain an insertion loss equal to or less than 2 dB whentaking into consideration the insertion loss, the thickness (h_(ion)) ofthe ion implantation layer is needed to satisfy the condition0.07<h_(ion) /λ_(w) <0.30. When using Ar ions, ion implantation can beeasily carried out by means of an ordinary ion implantation apparatusrather than a specific one when the thickness (h_(ion)) of the ionimplantation layer is needed to satisfy the condition 0.07<h_(ion)/λ_(w) <0.15.

A description will now be given of the relationship between the dose ofions and the filter characteristics. FIGS. 14A through 14F are graphsshowing the pass-band characteristics of the SAW filter shown FIGS. 8Aand 8B as a function of the dose of Ar ions. In FIGS. 14A through 14F,the acceleration energy of Ar ions (Ar⁺⁺) is 180 keV, and the doses ofAr ions are as follows:

FIG. 14A: 2×10¹³ ions/cm² ;

FIG. 14B: 4×10¹³ ions/cm² ;

FIG. 14C: 5×10¹³ ions/cm² ;

FIG. 14D: 6×10¹³ ions/cm² ;

FIG. 14E: 7×10¹³ ions/cm² ; and

FIG. 14F: 8×10¹³ ions/cm².

The values of the shape factor (Δf₂ /Δf₁) of the samples respectivelyrelated to FIGS. 14A through 14F are varied as the function of the doseof Ar ions as follows:

FIG. 14A: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.95

FIG. 14B: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.92

FIG. 14C: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.67

FIG. 14D: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.69

FIG. 14E: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.74

FIG. 14F: .sub.Δ f₂ /.sub.Δ f₁ =approximately 1.67

It can be seen from the above that the in-pass-band characteristicsshown in FIGS. 14C, 14D and 14E are approximately flat, whereas thein-pass-band characteristics shown in FIGS. 14A, 14B and 14F are notflat and does not have good linearity.

As can be seen from the above, the dose of Ar ions can be equal to orgreater than 4×10¹³ ions/cm² taking into consideration only the shapefactor. However a dose of 8×10¹³ ions/cm² should be excluded taking intoconsideration the pass-band characteristic. Hence, it is concluded,taking into account the practical insertion loss, that the dose of Arions is between 4×10¹³ ions/cm² and 7×10¹³ ions/cm², preferably between5×10¹³ ions/cm² and 7×10¹³ ions/cm². The pass-band characteristicobtained at the dose of Ar ions within the above range is not variedwith the range of the ion acceleration energy between 150 and 250 keV.Hence, it is recommendable that the dose of Ar ions is between 150 and250 keV.

In the above-mentioned ion implantation, there is a possibility thatcharges may be stored the piezoelectric substrate 1 or the piezoelectricthin film provided on the predetermined base, which may be thendischarged due to pyroelectricity caused by the piezoelectricity of thepiezoelectric member and that the piezoelectric substrate or thin filmmay be destroyed.

With the above in mind, means for eliminating the above possibility asshown in FIGS. 15A, 15B, 16A and 16B can be employed. In FIG. 15A, ametallic film 31 is coated on the back surface of the piezoelectricsubstrate 1. Then, as shown in FIG. 15B, the ion implantation layer 11is formed in the front surface portion of the piezoelectric substrate 1having the back surface on which the metallic film 31 is coated.Alternatively, as shown in FIG. 11A, a metallic plate 32 is provided onthe back surface of the piezoelectric substrate 1. Then, as shown inFIG. 11B, the ion implantation layer 11 is formed in the front surfaceportion of the piezoelectric substrate 1 having the back surface onwhich the metallic plate 32 is provided.

A further description will now be given of other applications of SAWdevices to SAW resonator type filters.

The period (electrode pitch) λ of the comb-type electrode is defined asfollows:

    λ=V/fo

where V is the surface acoustic velocity of the SAW and fo denotes theresonance frequency. The surface acoustic velocity V depends on theweight of the electrode, thus the thickness thereof. More particularly,the weight of the electrode becomes greater, the surface acousticvelocity V becomes slower. This is called a mass load effect.

Normally, the SAW resonator-type filter includes SAW resonators ofdifferent resonance frequencies which are connected in series andparallel, namely, in a ladder formation. It is possible to obtaindesired filter characteristics of the filter by adjusting the resonancefrequency interval .sub.Δ f, which is the difference between theresonance frequency (f_(s)) of the series-connection (series-arm)resonator and that (f_(p)) of the parallel-connection (parallel-arm)resonator (.sub.Δ f=f_(s) -f_(p)). It will be noted that the resonancefrequency of the series-connection resonator is approximately equal tothe anti-resonance frequency of the parallel-connection resonator.However, there is a limit regarding adjustment of the resonancefrequency interval .sub.Δ f. If the resonance frequency interval exceedsa limited value, the filter characteristics will deteriorate greatly.

The resonance frequency interval .sub.Δ f, which determines the filtercharacteristics, depends on the difference .sub.Δ λ between the periodof the comb-type resonator connected in series and the period of thecomb-type resonator connected in parallel. The period difference .sub.Δλ is limited by the electromechanical coupling factor k². If the perioddifference .sub.Δ λ is too small or too large, the SAW resonator-typefilter will provide a great insertion loss.

As has been described previously, the electromechanical coupling factork² can be controlled by ion implantation and adjustment of the heattreatment condition. This ion implantation functions to relax the abovelimitation regarding the period difference .sub.Δ λ and to increase thedegree of freedom in design of the pass-band of the filter. The heattreatment is not limited to a condition such that the heat treatmenttemperature is equal to or higher than 350° C. as described previously,but utilizes the fact that the SAW propagation speed is varied by theheat treatment.

Normally, the lines (electrode finger portions) and spaces of thecomb-type electrode between two adjacent lines are arranged at a ratioof approximately 1:1, although there are errors due to the precision ofthe exposure process and the precision of a mask used in the exposure.

For example, a wide-band filter in the 1.5 GHz range is achieved by thefollowing parameters:

Substrate: 36°-Y-cut X propagation LiTaO₃

(surface acoustic velocity:approximately 4000 m/s; k² =5%)

Period of comb-type electrode in series-connection resonator_(s) : 2.60μm

Period of comb-type electrode in parallel-connection resonator_(p) :2.70 μm

The period difference .sub.Δ λ (=λ_(p) -λ_(s)) is 0.1 μm, and a filtercharacteristic is obtained in which the resonance and anti-resonancefrequencies are 60 MHz and the 3 dB band range is approximately 40 MHz.

A low-insertion-loss narrow band filter can be obtained by utilizing apiezoelectric substrate having a small electromechanical coupling factork², for example, an ST cut substrate of quartz (the surface acousticvelocity: approximately 3000 m/s; k² :0.16

It has been confirmed from our experimentation that the interval betweenthe resonance frequency and the anti-resonance frequency of the abovequarts ST-cut substrate is between 2 MHz and 6 MHz, and is 1/10 to 1/30as large as that of the 36°-Y-cut X-propagation substrate of LiTaO₃.

From the above description, it is possible to achieve a narrow-bandfilter having the pass-band width between 3 MHz and 4 MHz by thecombination of resonators respectively having an electrode pitch λ_(s)of 1.995 μm and λ_(p) of 2.000 μm (.sub.Δ λ=0.005 μm).

By the way, a reduced projection exposure method is used to form finepatterns. For example, in order to form the above narrow-band filter bymeans of a reticle of a magnification of five, the width of each line ofthe electrode on the reticle used to realize the series-connection andparallel-connection resonators formed on the quartz substrate isrespectively 2.49375 μm and 2.500 μm.

Nowadays, the reticle pattern is formed by electron beam exposure. Theminimum resolution level currently available in an electron beamexposure apparatus is approximately 0.1 μm. Hence, it is impossible tocorrectly the electrode line width 2.49375 μm. Hence, there is an error(rounding error) in the electrode line width, and the error decreasesthe Q value of the filter. This means that it is very difficult torealize narrow-band filters in the high frequency range.

A second embodiment of the present invention which will be describedbelow utilizes the concept of the aforementioned first embodimentthereof, and is intended to achieve high-Q SAW resonator-type filterswhich have a high Q value and can be produced by the existing electronexposure process. As will be described later, the interval between theseries-connection resonator and the parallel-connection resonator of thefilter can be finely adjusted.

FIG. 17 is a plan view of a SAW resonator-type filter according to thesecond embodiment of the present invention. In FIG. 17, parts that arethe same as those shown in FIGS. 3A through 3D are given the samereference numbers.

The SAW resonator-type filter shown in FIG. 17 is made up of the quartzsubstrate 1, a conductive wiring line 2, an input terminal 3, an outputterminal, a series-connection resonator 5 having an interdigitalelectrode 5A and an ion implantation layer 5B (hatched area), aseries-connection resonator 6 having an interdigital electrode 6A and anion implantation layer 6B (hatched area), parallel-connection resonators7 and 8 respectively having interdigital electrodes 7A and 8B, andground terminals 9 and 10.

The SAW resonator-type filter shown in FIG. 17 is characterized in thatthe ion (for example, Ar ion) implantation layers 5B and 6B includingthe electrodes 5A and 6A and their vicinity areas are formed in theseries-connection resonators 5 and 6 only in order to reduce the surfaceacoustic velocity propagating through the ion implantation layers 5B and6B, so that the resonance frequency interval .sub.Δ f can be adjusted.

According to our experiment, a variation in the surface acousticvelocity V equal to 99.2% is obtained under the following conditions:

Implanted ion: Ar ion

Acceleration energy: 180 keV

Dose: 1×10¹⁴ ions/cm⁻².

Hence, by implanting ions in portions including the series-connectionresonators 5 and 6 and determining the following parameters,

Period of comb-type electrodes in series-connection resonators 5 and 6λ_(s) : 1.98 μm

Period of comb-type electrodes in parallel-connection resonators 7 and 8λ_(p) : 2.00 μm it is possible to obtain filter characteristicsequivalent to those of a SAW resonator-type filter having no ionimplantation layers and the following parameters:

Period of comb-type electrodes in series-connection resonators 5 and 6λ_(s) : 1.996 μm

Period of comb-type electrodes in parallel-connection resonators 7 and 8λ_(p) : 2.000 μm

It will be noted that the period 1.98 μm of the comb-type electrodes inthe series-connection resonators 5 and 6 is lower than the limitregarding the formation of fine patterns in the lithography techniquebecause patterns having a size five times the above period are formedthe reticle on which patterns are formed by the electron beam exposuretechnique.

It is possible to adjust the frequency of the overall filter byimplanting ions in the series-connection resonators and/orparallel-connection resonators after the ion implantation layers 5B and6B are formed. By the above frequency adjustment, it is possible tocorrect a frequency deviation caused by a factor of the productionprocess.

FIGS. 18A through 18D are cross-sectional views of the method ofproducing the SAW resonator-type filter shown in FIG. 17. Thecross-sections shown in FIGS. 15A through 15B are taken along lineII--II shown in FIG. 17. The steps shown in FIGS. 18A through 18D arealmost the same as those shown in FIGS. 3A through 3D, respectively. Theheat treatment shown in FIG. 3C is not necessary to produce the SAWresonator-type filter according to the second embodiment of the presentinvention. However, the heat treatment provides various advantages, ashas been described previously. Hence, it is preferable to form the heattreatment layer 12 by heating the ion implantation layer 11.

The previous descriptions given with reference to the previouslydescribed figures hold true for the SAW resonator-type filter shown inFIG. 17. In short, when Ar ions are used, it is preferable to employ thefollowing conditions. That is, the dose of Ar ions can be equal to orgreater than 4×10¹³ ions/cm² taking into consideration only the shapefactor. However a dose of 8×10¹³ ions/cm² should be excluded taking intoconsideration the pass-band characteristic. Hence, it is concluded,taking into account the practical insertion loss, that the dose of Arions is between 4×10¹³ ions/cm² and 7×10¹³ ions/cm², preferably between5×10¹³ ions/cm² and 7×10¹³ ions/cm². The pass-band characteristicobtained at the dose of Ar ions within the above range is not variedwith the range of the ion acceleration energy between 150 and 250 keV.Hence, it is recommendable that the ion acceleration energy of Ar ionsis between 150 and 250 keV.

It is also preferable to control the ion implantation depth. As has beendescribed previously, it is preferable to control the thickness(h_(ion)) of the ion implantation layer satisfies the condition0.07<h_(ion) /λ_(w) <0.33.

In FIG. 19 is a plan view of a practical example of the secondembodiment of the present invention, and FIG. 20 is an equivalentcircuit diagram of the filter shown in FIG. 19. As shown in FIGS. 19 and20, the filter is made up of three series-connection (series-arm)resonators S and two parallel-connection (parallel-arm) resonators P1and P2. L1 through L4 respectively denote wires shown in FIG. 17, andinductances shown in FIG. 18. The ion implantation process was carriedout for the series-connection resonators S only. The period of the twocomb-type electrodes of each of the series-connection resonators S is2.64 μm, the aperture length is 60 μm, and the number of finger pairsare 115. The period of the two comb-type electrodes of theparallel-connection resonator P1 is 2.71 μm, the aperture length is 100μm, and the number of finger pairs are 80. The period of the twocomb-type electrodes of the parallel-connection resonator P2 is 2.71 μm,the aperture length is 100 μm, and the number of finger pairs are 40.

A description will now be given, with reference to FIG. 21, of a SAWresonator-type filter according to a third embodiment of the presentinvention. The SAW filter shown in FIG. 21 includes a quarts substrate,a wiring line 42, an input terminal 43, a series-connection resonator 44including an interdigital electrode 45 made up of two comb-typeelectrodes, and a parallel-connection resonator 46 including aninterdigital electrode 47 made up of two comb-type electrodes.

FIG. 22A shows a part of the interdigital electrode 47 of theparallel-connection resonator 46, and FIG. 22B shows a part of theinterdigital electrode 45 of the series-connection resonator 44. Theinterdigital electrode 47 has lines 47A and spaces 47B. Similarly, theinterdigital electrode 45 has lines 45A and spaces 45B. In FIG. 22A, thesymbol "L" denotes the width of the lines 47A and the symbol "S" denotesthe width of the spaces 47B. In FIG. 22B, the symbol "L" denotes thewidth of the lines 46A and the symbol "S" denotes the width of thespaces 46B.

It can be seen from FIGS. 22A and 22B that the ratio L:S of theparallel-connection resonator 46 is set to 1:1 (for example, 0.4 μm:0.4μm), and the width of each line 45A is less than that of each line 47A.

Our experiment shows that the surface acoustic velocity becomes fasterby 3% when the ratio L:S of the series-connection resonator 44 is set to0.91:1.09 (for example, 0.36 μm:0.44 μm).

The crystal substrate having the above series-connection resonator 44and the parallel-connection resonator 46 respectively having thefollowing ratios has a filter characteristic equivalent to a filter suchthat λ_(p) =2.000 μm and λ_(s) =1.994 μm:

L:S in parallel-connection resonator=0.40 μm:0.40 μm

L:S in series-connection resonator=0.36 μm:0.44 μm.

FIG. 23 is a graph of a filter characteristic of the filter shown inFIG. 21 having the above-mentioned parameter values. In FIG. 23, thevertical axis denotes the insertion loss (dB), and the horizontal axisdenotes the frequency (MHz). It can be seen from the graph of FIG. 23that the interval .sub.Δ f between the resonance frequency of theseries-connection resonator and the resonance frequency of theparallel-connection resonator can be controlled by varying the surfaceacoustic velocity and a sufficiently narrow band characteristic can berealized.

FIGS. 24 and 25 show a practical example of the SAW filter according tothe present invention. FIG. 26 is an equivalent circuit diagram of theSAW filter shown in FIGS. 24 and 25. The SAW filter shown in FIGS. 24and 25 includes a ceramic package 51, a filter chip 52 and a lid 53functioning as the ground. The ceramic package 51 is made of aluminaceramic, and dimensions of 3.8 mm×3.8 mm×1.5 mm (height). Electrodeterminals 54₋₁ through 54₋₆ made of Au are formed on the ceramic package51. The filter chip 52 is made of LiTaO₃, and dimensions of 2 mm×1.5mm×0.5 mm (thickness). The thickness of ion implantation layers formedin the surface portion of the filter chip 52 is 2200 Å.

On the surface of the filter chip 52, there are provided resonators R1through R5 having a comb-type electrode structure made of Al-2%Cu,having 100 finger pairs, an aperture length of 80 μm, a wavelength _(w)of 2.7 μm and a thickness of 1900 Å. The resonators R1 through R5 arearranged so as not to share SAW propagation paths thereof. Further,signal terminals 55₋₁ and 55₋₂ and ground terminals 55₋₃, 55₋₄ and 55₋₅are provided on the surface of the filter chip 52. Bonding wires 56₋₁through 56₋₅, which are made of Al or Au and have a diameter of 25 μm,are bonded to the terminals 54₋₁ through 54₋₅ and the terminals 55₋₁through 55₋₅. The bonding wire 56₋₃ connects the electrode terminals54₋₃ and 55₋₃ together. The bonding wire 56₋₄ connects the groundterminals 54₋₄ and 55₋₄ together. The bonding wire 56₋₅ connects theground terminals 54₋₅ and 55₋₅.

The bonding wires 56₋₃ through 56₋₅ are as long as 2.0 mm, and functionsas inductance elements L1, L2 and L3 shown in FIG. 26.

The ion implantation layers having a thickness of 2200 Å are formed withrespect to the series-arm resonators R2 and R4 in the aforementionedmethod. Alternatively, it is possible to implant ions in surfaceportions of the filter chip 52 on which the parallel-connectionresonators R1, R3 and R5 are formed. Of course, it is possible toexecute the heat treatment of the ion implantation layers.

FIG. 27 shows another practical example of the SAW filter according tothe present invention. A SAW filter 60 shown in FIG. 27 has apiezoelectric substrate of LiTaO₃ (36° Y-cut) having dimensions showntherein. There are arranged, from the input side, a parallel-connection(parallel-arm) resonator Rp1, a series-connection (series-arm) resonatorRs1, a parallel-connection resonator Rp2, a series-connection resonatorRs2 and a parallel-connection resonator Rp3. Each of the resonators hasreflectors (shortcircuit type) 62 arranged at opposite sides thereof.Each of the electrodes of the resonators has 50 finger pairs, anaperture length of 180 μm. The reflectors has 50 finger pairs. Theperiod of the comb-type electrodes of the parallel-connection resonatorsis different from that of the comb-type electrodes of theseries-connection resonators. For example, the period of theparallel-arm resonators is 4.39 μm (the pattern width is approximately1.1 μm (=λ_(p) /4) because the ratio of the pattern width and the spaceis 1:1). The period λ_(s) of the series-connection resonators is 4.16 μm(the pattern width is approximately 1.04 μm (=λ_(s) /4).

The ion implantation process and the heat treatment have been performedwith respect to surface portions of the piezoelectric substrate 60 onwhich the series-connection resonators Rsl and Rs2. Alternatively, it ispossible to perform the ion implantation process and the heat treatmentwith respect to surface portions on which the parallel-connectionresonators Rp1, Rp2 and Rp3.

In the aforementioned SAW filters, it is possible to perform the ionimplantation process and the heat treatment with respect to both theseries-connection resonators and the parallel-connection resonators atdifferent doses of ions to be implanted. In this case, it is possible tovery finely adjust the filter characteristic. For example, in FIG. 17,ions are implanted in the surface portions on which theseries-connection resonators 5 and 6 are formed at a dose different fromthat at which ions are implanted in the surface portions on which theparallel-connection resonators 7 and 8 are formed.

Alternatively, it is possible for the surface portions on which theseries-connection resonators 5 and 6 are formed to be implanted by akind of ions different from that of ions which are implanted in thesurface portion on which the parallel-connection resonators 7 and 8 areformed. It is also possible to control both the ion kind and the dose.

It is also possible to combine the concept of the ion implantationprocess directed to varying the surface acoustic velocity with theconcept of adjusting the line/space ratio. For example, it is possibleto form a SAW filter having the structure shown in FIG. 17 and thestructure shown in FIG. 21.

According to the present invention, it is possible to obtain thefollowing advantages.

It is possible to control the SAW propagation speed and theelectromechanical coupling factor by the ion implantation process andthe subsequent heat treatment independently of the piezoelectricsubstrate material and the crystal orientation, thus the SAW mode andthe temperature coefficient. In an application to SAW filters, it ispossible to provide SAW filters having a high Q-value and a high shapefactor by the ion implantation process and preferably the heat treatmentrather than selection of the piezoelectric substrate material as well asthe crystal orientation. In this case, it is possible to finely adjustthe filter characteristic by selectively implanting ions in surfaceportions on which resonators are formed. The fine adjustment of thefilter characteristic can be performed so that different kinds of ionscan be implanted with respect to different types of resonators. It isalso possible to implant ions with respect to different types ofresonators at different doses of ions.

By optimizing the thickness of ion implantation layers, it is possibleto optimize a desired filter characteristic without increase in theinsertion loss.

By setting the line/space ratio of the comb-type electrodes of theparallel-connection resonator to be different from that of theseries-connection resonator, it is possible to provide SAW filtershaving a desired filter characteristic rather than selection of thepiezoelectric substrate material as well as the crystal orientation.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A method of producing a SAW (Surface AcousticWave) device, comprising the steps of:a) implanting ions in an entiresurface of a piezoelectric member of the SAW device, so that an ionimplantation layer is formed therein; b) performing a heat treatment ofthe piezoelectric member, so that a heat-treated ion implantation layeris formed in the entire surface of the piezoelectric member; and c)providing an electrode of a comb type resonator on the heat-treated ionimplantation layer.
 2. A method of producing a SAW (Surface AcousticWave) device, comprising the steps of:a) implanting ions in an entiresurface of a piezoelectric member of the SAW device, so that an ionimplantation layer is formed therein; b) performing a heat treatment ofthe piezoelectric member, so that a heat-treated ion implantation layeris formed in the entire surface of the piezoelectric member; and c)providing an electrode of a comb type resonator on the heat-treated ionimplantation layer, wherein said step a) comprises the step ofimplanting ions so that the heat-treated ion implantation layer has athickness h_(ion) that satisfies the following condition:

    0.07<h.sub.ion /λ.sub.w <0.33

where λ_(w) denotes the wavelength of a surface acoustic wave.
 3. Themethod as claimed in claim 2, wherein said step a) comprises the step ofimplanting Ar ions at a dose between 4×10¹³ and 7×10¹³ ions/cm².
 4. Themethod as claimed in claim 1, wherein said step a) comprises the step ofimplanting ions in a state in which a metallic member is provided on asurface of the piezoelectric member opposite said surface portionthereof.
 5. The method as claimed in claim 1, wherein said step b)comprises the step of heating the piezoelectric member at a temperatureequal to or higher than 350° C. and less than the Curie temperature ofthe piezoelectric substrate material.
 6. The method as claimed in claim1, wherein said step b) comprises the steps of:heating the piezoelectricmember at a rate of 3° C. per minute or less; keeping the piezoelectricmember at a constant temperature during a time between 30 minutes and 1hour; and cooling the piezoelectric member at a rate of 2° C. per minuteor less.
 7. A method of producing a SAW (Surface Acoustic Wave) deviceincluding a first resonator formed on a piezoelectric member, and asecond resonator formed on the piezoelectric member, the first resonatorhaving a resonance frequency approximately equal to an anti-resonancefrequency of the second resonator, said method comprising the stepsof:a) implanting ions in an entire surface of the piezoelectric memberso that an ion implantation layer is formed in the entire surfacetherein; and b) forming electrodes of a comb type of first and secondresonators on the piezoelectric member so that at least one of the firstand second resonators is provided on said ion implantation layer.
 8. Amethod of producing a SAW (Surface Acoustic Wave) device including afirst resonator formed on a piezoelectric member, and a second resonatorformed on the piezoelectric member, the first resonator having aresonance frequency approximately equal to an anti-resonance frequencyof the second resonator, said method comprising the steps of:a)implanting ions in an entire surface of the piezoelectric member so thatan ion implantation layer is formed in the entire surface therein; andb) forming electrodes of a comb type of first and second resonators onthe piezoelectric member so that at least one of the first and secondresonators is provided on said ion implantation layer, wherein:said stepa) comprises the steps of forming a first part and a second part of theion implantation layer, so that at least one of a dose of ions containedin the first part and a kind of the ions contained therein is differentfrom a dose of ions contained in the second part or a kind of the ionscontained in the second part; and said step b) comprises the steps offorming the electrodes so that the first resonator is formed on thefirst part of said ion implantation layer, and the second resonator isformed on the second part of said ion implantation layer.
 9. A method ofproducing a SAW (Surface Acoustic Wave) device including a firstresonator formed on a piezoelectric member, and a second resonatorformed on the piezoelectric member, the first resonator having aresonance frequency approximately equal to an anti-resonance frequencyof the second resonator, said method comprising the steps of:a)implanting ions in an entire surface of the piezoelectric member so thatan ion implantation layer is formed in the entire surface therein; andb) forming electrodes of a comb type of first and second resonators onthe piezoelectric member so that at least one of the first and secondresonators is provided on said ion implantation layer, wherein said stepa) comprises the step of implanting ions so that the ion implantationlayer has a thickness h_(ion) that satisfies the following condition:

    0.07<h.sub.ion /λ.sub.w <0.33

where λ_(w) denotes the wavelength of a surface acoustic wave.
 10. Themethod as claimed in claim 9, wherein said step a) comprises the step ofimplanting Ar ions at a dose between 4×10¹³ and 7×10¹³ ions/cm².
 11. Themethod as claimed in claim 7, further comprising the step c) ofperforming, before said step b), a heat treatment of the piezoelectricmember at a temperature higher than 350° C. and lower than the Curietemperature of the piezoelectric substrate material so that aheat-treated ion implantation layer can be formed.